Earlier studies, utilizing irradiation as the mutagenic source, have shown that the genome of the test organism is a major factor in the determination of its susceptibility to genetic damage. Preliminary screening of five inbred strains in our laboratory, using chemical mutagens indicate the existence of strain differences between these strains of mice. Future efforts will include retests of these strains plus additional strains. In addition, efforts will be devoted to understanding the underlying genetic causes of the observed differences.